High sensitivity electron beam magnetometer

ABSTRACT

An electron beam magnetometer having a sensitivity equal to 10 9 gauss or better comprising an evacuated envelope, means for generating a stream of electrons positioned in the interior thereof, control grid and focusing electrode means positioned downstream along the axis of the generated electrons, a pair of cores of a ferromagnetic material for densifying an induced magnetic flux positioned downstream of the focusing electrode means so as to form an air gap for the passage of the electron beam, deflection means located downstream of the magnetic flux densifying material, target means connected to suitable indicating means located downstream of the deflection means and final acceleration anode means located adjacent the target means. All of said components are operably connected to a suitable power source.

United States Patent Heidenwolf 1 Apr. 10, 1973 HIGH SENSITIVITYELECTRON BEAM OTHER PUBLICATIONS MAGNETOMETER Electronics; Apr. 3, 1967;pp. 270-271 [75] Inventor: Hermann Heidenwolf, Adnet, Kliever t AIEETrans 1947; pp- Austria [73] Assignee: The United States of America asPrimary Examiner A1fred E Smith represented y the Secretary of theAttorneyHarry M. Saragovitz, Edward J. Kelly and Amy Herbert Berl [22]Filed: Jan. 25, 1971 [57] ABSTRACT [21] Appl' No" 109,240 An electronbeam magnetometer having a sensitivity equal to 10' gauss or bettercomprising an evacuated [52] US. Cl. ..324/44 envelope, means forgenerating a stream of electrons [51] Int.Cl. ..G01r 33/02 positioned inthe interior thereof, control grid and [58] Field of Search ..324/44;313/89, 72 focusing electrode m n positioned wn ream along the axis ofthe generated electrons, a pair of [56] References Cited cores of aferromagnetic material for densifying an induced magnetic fluxpositioned downstream of the UNITED STATES PATENTS focusing electrodemeans so as to form an air gap for the passage of the electron beam,deflection means 3,657,643 4/1972 Nicholson ..324/44 located downstremnof the magnetic flux densifying 2358901 9/1944 Ziebolz "'"324/44 xmaterial, target means connected to suitable indicat- 2465277 3/1949Schafeh" "313/89 X ing. means located downstream of the deflection2562696 7/1951 X means and final acceleration anode means located ad-2,720,558 10/1955 Skellett ..324/44 jacent the target means. All of saidcomponents are operably connected to a suitable power source.

7 Claims, 6 Drawing Figures HIGH SENSITIVITY ELECTRON BEAM MAGNETOMETERThis invention relates to improvements in electron beam systems forresponding to magnetic fields and more particularly to a highsensitivity electron beam magnetometer.

In many instances, it is desired to produce an electrical signal orimpulse in accordance with relatively small changes in magnetic fields.For example, in detecting submarines from ships or airplanes, a deviceis desired which will produce a usable electric signal on the rathersmall change in local magnetic fields due to the relatively distant massof the submarine. A similar need arises in regard to military minesintended to set off explosives on approach of a vehicle or ship within apredetermined distance. Similar problems arise in the investigation ofmagnetic fields of metallic or non-magnetic specimens of the earthsfield as in prospecting. In the measurement of magnetic gradient, aninstrument sensitive to differences of magnetic fields is required.

The accuracy obtained to date with magnetometer systems such as theFoerster probe or the Rubidium vapor magnetometer is not sufficient tosatisfy progressing demands of modern magnetometry. For example, anymagnetic fields of the moon have as yet not been detected, although theearths magnetic field should still have some small effect there.

The present system operates on the principle that when a bar of aferromagnetic material is positioned in a magnetic field, a magneticflux is induced into the material. In the present system a ferromagneticmaterial of high permeability is encompassed in a vacuum tube downstreamof a cathode. Inoperation, generated electrons are accelerated andimpinged upon target means. The generated electrons are bent by themagnetic flux induced into the ferromagnetic material. In the absence ofa local magnetic field, the bent and accelerated electron beam isadjusted so that it impinges equally on the target means by theapplication of a potential to defecting means. This gives a zero readingon a suitably calibrated indicating means. In the presence of a localmagnetic field the generated electron beam impinges unequally on thetarget means. This gives a reading on suitable calibrated meansindicative of the local magnetic field.

It is an object of this invention to provide and disclose an improvedsystem adapted to respond to the gradient of a magnetic field.

It is a further object of this invention to provide and disclose animproved high sensitivity electron beam magnetometer having asensitivity equal to gauss or better.

It is a further object of this invention to provide and disclose animproved electron beam magnetometer wherein the sensitivity thereof isenhanced by a magnetic field concentrator in the vicinity of thecathode.

It is a further object of the invention to provide and disclose animproved electron beam magnetometer comprising a magnetic fieldconcentrator in the vicinity of the cathode which deflects theelectrons, thus shifting the locating of deflection being originallyfocused on both lenticular plates equally to prevalently one or theother of said lenticular plates.

It is a further object of this invention to provide and disclose animproved electron beam magnetometer comprising an electron beamaccelerator thereby necessitating higher voltage application on thedeflection means to deflect the electron beam, thus increasingsensitivity.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims taken in conjunctionwith the accompanying drawing in which:

FIG. 1 shows a cross-sectional elevated view, partially in schematic, ofthe present apparatus.

FIG. 2 is a section on line 33 of FIG. I.

FIG. 3 shows an alternative of FIG. 1 which comprises electromagneticmeans to deflect the electronic beam.

FIG. 4 shows an alternative of FIG. 1 which comprises magnetostaticmeans to deflect the electron beam.

FIG. 5 shows a schematic illustration of an alternative of FIG. 1 whichutilizes a Thermistor bridge to indicate the presence of the localmagnetic field.

FIG. 6 is a schematic illustration of an alternative of FIG. 1 whereinthe target means comprise a pair of thermocouples.

Referring now to FIG. 1 of the drawing, the system comprises elongatedtubular envelope l1. Said envelope may be constructed of glass or anyother suitable material. Cathode l3 and cathode shield 15 are positionedin the interior of envelope 11. Control grid means 17 is positionedadjacent cathode shield 15. First anode and electronic lens 19, whichconstitutes the focusing electrode, is positioned adjacent control grid17 along the axis of tube 11. Upper pole shoe 21 and lower pole shoe 23constructed of high permeability ferromagnetic materials are positionedadjacent focusing electrode 19. Cores 21 and 23 comprise the internalmagnetic flux densifiers. Permalloy, a material composed of about 78percent nickel and 21 percent iron, was utilized. The pole pieces arespaced to form gap 25, which comprises the path of the passage ofelectron stream 26 between the ferromagnetic material. Electrostaticdeflection plates 27 and 29 are positioned downstream of theferromagnetic material on each side of the generated electron beam.Internal concentric densifier 31, which comprises a configurationconforming to tubular envelope 11 is positioned in the interior thereofdownstream of the cores of ferromagnetic materials. External concentricdensifier 33, which also comprises a configuration conforming to tubularenvelope 11, is positioned on the exterior thereof upstream from theposition of internal densifier 31. The internal and external densifiersmay be constructed of a material identical to ferromagnetic materials 21and 23. Lenticular collector plates 35 and 36 are positioned downstreamfrom the electrostatic deflection plates. The collector plates areconnected to any suitable indicating means, e.g., galvanometer 37.

In operation, the system is positioned in any direction relative to thelocal magnetic field. Electrons emitted by cathode 13 are acceleratedand focused into electron beam 26 along the axis of the tube byelectrode 19 positioned in the tube and maintained at a positivepotential with respect to cathode 13 by means of a suitable source ofpotential represented by battery 39. A potential of 400 volts, forexample, is applied to electrode 19 with respect to cathode 13. Thevoltage is controlled and measured by means of potentiometer 43. Amagnetic flux is induced into ferromagnetic masses 21 and 23 by theearth's magnetic field and any other incidental or local magnetic field.Internal and external concentric magnetic densifiers 31 and 33 aid inchanneling the magnetic flux into the path of the columnated electronbeam. Densifiers 31 and 33 are tubular in configuration, each with anangular end terminating with a plate that overlies ferromagnetic masses21 and 23, respectively. Densifier 31 is in physical contact withferromagnetic mass 21, and densifier 33 is magnetically coupled toferromagnetic mass 23. Together, the densifiers with their correspondingplates and masses form a dipole antenna which collects and concentratesthe magnetic flux to be detected in gap 25. Since the potential on lowacceleration electrode 19 is very low compared to the 10,000 volts onanode 38, the electron beam passing through gap 25 is, comparatively,drifting slowly and thus is very sensitive to the modulations imposedthereon by the highly densified received magnetic flux. Since masses 21and 23 are connected to low acceleration electrode 19, the extremelyhigh acceleration effect of cathode 38 is not felt until the cathodebeam emerges from gap 25. This feature enables the beam to be excited atlow velocity resulting in a high amplification factor, and then beaccelerated to a high velocity for further amplification. Generatedelectron beam 26 is bent by the magnetic flux as it passes through gap25, accelerated to a high velocity by anode 38, and impinged uponcollector plates 35 and 36. Anode 38 is maintained, for example, at apotential of 10,000 volts with respect to the cathode. Deflecting plates27 and 29 serve as null adjustors to compensate for the magnetic fluxinduced into ferromagnetic masses 21 and 23 by the earths magneticfield. In operation, a potential is'applied to the deflecting platessufficient to suppress the effect of the earth s magnetic field. This isindicated by impingement of the electron stream equally on bothlenticular plates. When a local magnetic field is induced into thesystem, and the apparatus has been corrected to obviate the effect ofthe earths magnetic field, the electron stream impinges unequally onboth target plates. Calibrated means indicate the intensity of the localmagnetic field. The present combination results in a magnetometer havinga sensitivity of 0.0001 gamma which is equal to gauss or better.

The lenticular collector plates are illustrated by FIG. 2. The platesmay be coated with a fluorescent powder to make the electron beamimpinging on the plates visible for easier compensation.

FIG. 3 illustrates a method for deflecting the electron beam tocompensate for the earths magnetic field by means of magnetic deflectingcoil 45 in lieu of electrostatic deflecting plates 27 and 29. The coilis energized by any suitable power source, not shown. Coil 45establishes a magnetic field traversely of beam 26 whereby the beam isdeflected in a direction depending upon the direction of the fieldestablished by the coil.

FIG. 4 illustrates a method for deflection of the electron beamcomprising small permanent magnets producing magnetic field 47 which ismechanically moved with respect to the electron beam.

The Thermistor bridge illustrated in FIG. 5 obviates the need forlenticular collector plate means. Thermrstors are thermally sensitiveresistors whose primary function is to show a change in temperatures.The present bridge comprises therrnistors 49 and 51, meter 53, variableresistor 55, resistor 57 and a power source, e.g., battery 59. After thesystem has been adjusted to a zero reading, and in the presence of alocal magnetic field, the generated electrons impinge unequally on thethermistors 49 and 51 which unbalances the bridge thereby allowing ameasurable flow of current as indicated on calibrated meter 53.

FIG. 6 illustrates an alternative of FIG. 1 wherein thermocouple means61 and 63, connected to a suitable indicator, designated 65, areutilized as the target means.

Although I have described my invention with a certain degree ofparticularity, it is understood that the present disclosure has beenmade by way of example and that numerous changes in the details ofconstruction and combination and arrangement of parts may be resorted towithout departing from the spirit and the scope of the invention.

Having described my invention, I claim:

1. An electron beam magnetometer having a sensitivity equal to 10' gaussor better, comprising an evacuated envelope, cathode means encompassedtherein for generating an electron beam, low potential focusing anodemeans positioned downstream of the cathode means, a first magnetic fluxreceiver and densifier means connected to a first ferromagnetic mass,and a second magnetic flux and densifier means magnetically coupled to asecond ferromagnetic mass, means between said masses defining a narrowgap for the passage of said electron beam, said ferromagnetic massesconnected to said focusing anode, deflection means located downstream ofthe magnetic flux densifier means, target means connected to suitableindicating means located downstream of the deflection means, relativelyvery high potential accelerating anode means located adjacent the targetmeans, all of said component operably connected to a suitable powersource.

2. An apparatus in accordance with claim 1 wherein the deflection meanscomprises a pair of electrostatic deflection plates located in theinterior thereof.

3. An apparatus in accordance with claim 1 wherein the deflection meanscomprises an energized coil positioned on the exterior of the envelope.

4. An apparatus in accordance with claim 1 wherein the deflection meanscomprises an external magnetic field positioned adjacent the envelope.

5. An apparatus in accordance with claim 1 wherein the target meanscomprises a pair of lenticular plates.

6. An apparatus in accordance with claim 1 wherein the target comprisesa pair of thermistor means connected to a suitable power source andindicator.

7. An apparatus in accordance with claim 1 wherein the target comprisesthermocouples means connected to a suitable indicator.

1. An electron beam magnetometer having a sensitivity equal to 10 9gauss or better, comprising an evacuated envelope, cathode meansencompassed therein for generating an electron beam, low potentialfocusing anode means positioned downstream of the cathode means, a firstmagnetic flux receiver and densifier means connected to a firstferromagnetic mass, and a second magnetic flux and densifier meansmagnetically coupled to a second ferromagnetic mass, means between saidmasses defining a narrow gap for the passage of said electron beam, saidferromagnetic masses connected to said focusing anode, deflection meanslocated downstream of the magnetic flux densifier means, target meansconnected to suitable indicating means located downstream of thedeflection means, relatively very high potential accelerating anodemeans located adjacent the target means, all of said component operablyconnected to a suitable power source.
 2. An apparatus in accordance withclaim 1 wherein the deflection means comprises a pair of electrostaticdeflection plates located in the interior thereof.
 3. An apparatus inaccordance with claim 1 wherein the deflection means comprises anenergized coil positioned on the exterior of the envelope.
 4. Anapparatus in accordance with claim 1 wherein the deflection meanscomprises an external magnetic field positioned adjacent the envelope.5. An apparatus in accordance with claim 1 wherein the target meanscomprises a pair of lenticular plates.
 6. An apparatus in accordancewith claim 1 wherein the target comprises a pair of thermistor meansconnected to a suitable power source and indicator.
 7. An apparatus inaccordance with claim 1 wherein the target comprises thermocouples meansconnected to a suitable indicator.